1. Field
This invention is in the field of pressure measurement devices and particularly devices such as rheometer plates for measuring local pressures of fluids during shear and restricted flow.
2. State of the Art
Normal stress differences and viscosities are important properties of plastics and other liquids that need to be characterized and taken into account for the efficient processing of these liquids. One way to measure normal stress differences is to measure the local pressure of the fluid under shear conditions against a rheometer plate. Fluid is sheared between one stationary plate and a rotating cone or plate. The stationary plate is connected to a spring, which deflects in response to the normal stress difference of the liquid being sheared. However, with this method, the deflection of the spring results in a change of the gap between the stationary plate and the rotating cone. For accurate measurement, this gap should be held constant during measurement. In order to minimize the change in the gap, a substantially stiff spring has been employed. The stiff spring, however, requires a very elaborate detection circuit for accurate detection of small deflection. A servo system may be employed to compensate for the deflection automatically. However, the finite response time of the feedback control of the servo system is a source of uncertainty especially under a dynamic control system. The accuracy of the detection circuit sets up a limit on minimum size of the rheometer plate, which determines the amount of deflection of the spring. In other words, the accuracy of the normal stress difference measurement depends on the size of the plate and the detection circuit accuracy.
Alternatively, rather than using a spring mounted rheometer plate and measuring the displacement of the plate under pressure of the liquid against the plate, a fixed rheometer plate with two or more pressure sensors attached to the plate can be used to measure pressure exerted by the liquid during shear at the locations on the plate where the pressure sensors are located. This alternative method is described in Ramachandran, S. and E. B. Chriatiansen, AIChe 1985, 31, 162; Seong-Gi Baek, Ph.D. Thesis, University of Utah, 1991; and S. G. Baek, J, J, Magda, S. Cementwala, J. of Rheology, 1993, 37 (5), 935. As described in these references, individual pressure transducers are mounted to a stationary plate. The sensing diaphragms of individual pressure transducers, however, should be flush mounted with the stationary surface in order not to disturb flow due to the roughness generated otherwise. Thus, the mounting of the individual transducers has to be done carefully to minimize surface roughness. However, it is generally impossible to remove such surface roughness completely with sensors individually mounted in the surface, so some effect from the roughness will always be present.
With the type of rheometer described above, setting the right distance between the cone and the plate and keeping it during measurement is critical for accuracy. With current rheometers, the distance is set mechanically. First the cone is lowered until it touches the plate. Then the cone is raised to the specified distance from the point of contact for a gap. Current rheometers, however, do not provide any mean to monitor the position of the cone during measurement.
Rather than using a rheometer plate with a rotating cone creating the shear in the liquid to be measured, the liquid can be forced through a flow passage and the pressure exerted on the walls of the passage measured similarly to the pressure on the rheometer plate. Generally, the liquid will be forced through a passage having set dimensions and the pressure exerted against a wall of the passage will be measured by pressure transducers set in the wall. Again, it is important to keep the wall as smooth as possible. Using a flow passage with set dimensions can provide a measurement of the apparent viscosity of the liquid. If the liquid is then forced through a second device with a flow passage of different set dimensions, the pressure measurements obtained can be compared to the first measurements and the exact viscosity calculated.
U.S. Pat. No. 5,983,727 shows a device for measuring the pressure of liquids against a surface of the device. The device has a mounting structure with a recess therein. An elastic membrane extends over the recess and at least one transducer is mounted in the recess to detect deflection of the membrane at a selected plurality of regions on the membrane. However, relatively complex sensors are required to measure deflection at a plurality of regions on the membrane or to separate measurements made by several sensors in the same recess measuring different regions of the membrane.